Allow reactivate a user without password

This issue has been migrated from [#16739](https://github.com/matrix-org/synapse/pull/16739).

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Closes: #10397

An administrator should know whether he wants to set a password or not.
There are many uses cases where a blank password is required.

- Use of only some users with SSO.
- Use of bots with password, users with SSO

### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog file](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#changelog). The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
* [x] Pull request includes a [sign off](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#sign-off)
* [x] [Code style](https://matrix-org.github.io/synapse/latest/code_style.html) is correct
(run the [linters](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

Signed-off-by: Dirk Klimpel [5740567+dklimpel@users.noreply.github.com](mailto:5740567+dklimpel@users.noreply.github.com)